1. Field of the Invention
The invention relates to the process of fabricating semiconductor chips. More specifically, the invention relates to a method and an apparatus for assigning different phases to selected cuts on a complementary mask to clear phase conflicts generated by a phase shifting mask during an optical lithography process used in fabricating a semiconductor chip.
2. Related Art
Recent advances in integrated circuit technology have largely been accomplished by decreasing the feature size of circuit elements on a semiconductor chip. As the feature size of these circuit elements continues to decrease, circuit designers are forced to deal with problems that arise as a consequence of the optical lithography process that is typically used to manufacture integrated circuits. This optical lithography process begins with the formation of a photoresist layer on the surface of a semiconductor wafer. A mask composed of opaque regions, which are generally formed of chrome, and light-transmissive clear regions, which are generally formed of quartz, is then positioned over this photoresist layer. (Note that the term xe2x80x9cmaskxe2x80x9d as used in this specification is meant to include the term xe2x80x9creticle.xe2x80x9d) Light is then shone on the mask from a visible light source, an ultraviolet light source, or more generally some type of electromagnetic radiation source together with suitably adapted masks and lithography equipment.
This image is reduced and focused through an optical system containing a number of lenses, filters, and mirrors. The light passes through the clear regions of the mask and exposes the underlying photoresist layer. At the same time, opaque regions of the mask block the light leaving underlying portions of the photoresist layer unexposed.
The exposed photoresist layer is then developed, through chemical removal of either the exposed or non-exposed regions of the photoresist layer. The end result is a semiconductor wafer with a photoresist layer having a desired pattern. This pattern can then be used for etching underlying regions of the wafer.
Printing Problems with Cuts in Close Proximity
As integration densities continue to increase, it is becoming desirable to use phase shifters to define more and more features within a layout. This can lead to problems in some situations. For example, FIG. 1A illustrates the phase shifters for a dark field alternating aperture phase shifting mask 100. The phase shifters (shown interposed against the original layout shown with a lighter stipple than the rest of the field and with slanted lines to indicate phase shifters) are set on a dark field, e.g. chromium. The lightly stippled areas between the phase shifters correspond to the intended, or original layout.
Referring to FIG. 1A, a desired feature 102 can be formed using zero-degree phase shifter 104 and 180-degree phase shifter 106. At the right end of feature 102, there is a small region 108, or cut, between phase shifters 104 and 106 that is not part of the layout and is not intended to print. Region 108 will leave a dark area that must be cleared by a complementary, or trim, mask during the optical lithography process, additionally there may be a loss of definition in the areas surrounding this region.
A complementary mask 101 (also known as a trim mask) provides a corresponding cut 110 at the appropriate location to fully expose the photoresist in region 108. Note that other cuts in complementary mask 101 expose other regions that are not intended to print between phase shifters on phase shifting mask 100.
In particular, note that cuts 112 and 114 are in close proximity to each other on the complementary mask 101. This can cause unwanted exposure in a region between cuts 112 and 114. This can be seen by examining FIG. 2A, which illustrates an aerial image 200 created by exposure through complementary mask 101. An areas"" intensity of exposure is shown using colors with the most intensity shown as red, orange, yellow, green, blue, and violet in descending intensity. Consider areas 202 and 204, which correspond to cuts 112 and 114, respectively, in complementary mask 101. Note that the close proximity of cuts 112 and 114 to each other causes unwanted exposure of the region between cuts 112 and 114.
This unwanted exposure results in a lack of definition in the printed image between cuts 112 and 114. This lack of definition is apparent in corresponding region 208 in FIG. 2B, which presents an aerial image 206 of how the layout will print. Notably, the lighter blue color in region 208 of the feature shows that the line may be broken. In FIGS. 2A and 2B, the exposure conditions used were xcex=193 nm, NA=0.85, "sgr"=0.4, dosage ratio 3:1 (trim:phase ratio in mJ/cm2).
Hence, what is needed is a method and an apparatus for clearing phase conflicts on a phase shifting mask without the problems described above.
One embodiment of the invention provides a system that assigns different phases to selected cuts on a complementary mask used to clear phase conflicts generated by a phase shifting mask during an optical lithography process used in fabricating a semiconductor chip. The system first receives the complementary mask, which uses cuts to clear phase conflicts generated by the phase shifting mask. Next, the system identifies the cuts on the complementary mask that may not clear the phase conflicts, and then assigns a different phase to selected cuts on the complementary mask, so that the selected cuts are out-of-phase with other cuts on the complementary mask. Assigning a different phase to the selected cuts ensures that the cuts on the complementary mask clear the phase conflicts generated by the phase shifting mask. And assists in the definition of the adjacent features.
In a variation of this embodiment, identifying the cuts that may not clear the phase conflicts involves locating cuts that are close enough to each other to cause unwanted exposure in the region between the cuts during the exposure through the complementary mask. In some instances the unwanted exposure may be an over exposure (e.g. feature definition suffers), in other instances the unwanted exposure may be an under exposure (e.g. conflict not fully cleared.)
In a further variation, the system varies the phase between the cuts to eliminate the unwanted exposure in the region between the cuts during the exposure through the complementary mask.
In a further variation, identifying the cuts that may not clear the phase conflicts involves locating isolated cuts that are too small to clear the phase conflicts.
In a further variation, assigning the different phase to selected cuts on the complementary mask involves adding out-of-phase assist features to the complementary mask in proximity to the isolated cuts.
In a further variation, identifying the cuts on the complementary mask that may not clear the phase conflicts involves simulating exposures through the phase shifting mask and the trim mask to produce a simulation result, and then examining the simulation results.